The present invention relates to a silica glass member, a method for using the same, and a projection aligner using the same. In further detail, the present invention relates to a silica glass member for use as a lens of imaging optics for patterning desired mask patterns on substrates by utilizing light sources in the ultraviolet region or vacuum ultraviolet region such as excimer laser radiations, a photomask such as a reticle for transferring circuit patterns of integrated circuits, a diffraction optical element (DOE), an Etalon plate for light sources, or a blank thereof, to a method for producing said silica glass member, and to a projection aligner using the same.
As a projection aligner for use in producing semiconductor devices, conventionally used are those having the structure shown in FIG. 16A and FIG. 16B.
More specifically, referring to a projection aligner 800 shown in FIG. 16A, a beam emitted from a light source 501 such as a mercury arc lamp, etc., is converged by an elliptical mirror 502, and is converted into parallel beams through a collimator lens 503. The parallel beams then pass through a fly-eye lens 504 consisting of an assembly of optical elements 504a each having a square cross section shown in FIG. 16B to form a plurality of light source images on the light emitting side. An aperture stop 505 having a round opening portion is provided at the position of the light source images. The light beams emitted from the plurality of the light source images are then converged by a condenser lens 506, and a reticle R, which is the object to be irradiated, is uniformly illuminated by the superimposed beams.
The pattern on the reticle R, which is uniformly illuminated by the illuminating optics as above, is projection aligned on a wafer W coated with a resist by a projection optical system 507 consisting of a plurality of lenses. The wafer W is mounted on a wafer stage WS provided movable in two dimension; in case of a projection aligner 800 shown in FIG. 16A, the alignment is performed in the so-called step-and-repeat system (stepper), in which the wafer stage is sequentially moved two-dimensionally for the exposure of the next shot region after the exposure is completed for a one-shot area on the wafer.
Recently, furthermore, there is proposed a scanning type aligner system capable of transferring the pattern of a reticle R onto a wafer W at high throughput, in which a square or an arc beam is irradiated to the reticle R while scanning, in a predetermined direction, the reticle R and the wafer W that are provided in a conjugate arrangement with respect to the projection optics 507.
In order to transfer finer mask pattern images on the wafer plane, i.e., to further increase the resolution, it is further proposed recently to shorten the wavelength of the light source. For instance, radiations with shorter wavelength such as KrF (248 nm) and ArF (193 nm) excimer lasers are being used instead of the conventionally used g-line (436 nm) or i-line (365 nm).
As an optical member for use in the optical system inside the projection aligners above, it is preferred that the member yields a high transmittance for the radiation having the wavelength corresponding to that of the light source being used in the system. This is because, since the optical system of the projection aligner is constructed of a plurality of optical members, even if the optical loss per 1 lens should be small, the total accumulation of the loss for the total number of the optical members leads to a large drop in transmittance. If an optical member having a poor transmittance is used, the exposure light is absorbed by the optical member as to increase the temperature thereof, and this results in a heterogeneous distribution in refractive index inside the optical member; furthermore, the local thermal expansion of the optical member leads to a deformation in the polished plane. Thus, a deterioration in optical performance is induced. In the case of a projection optical system, in particular, it is required that the optical member yields a highly uniform refractive index to obtain fine and clear projection exposure patterns, because a fluctuation in refractive indices leads to a retardation that greatly affects the imaging performance of the projection optical system.
In general, in case of a projection aligner using a light having a longer wavelength than an i-line, a optical glass made from multi-component optical glass is being used for the lens member of the illumination optical system or the projection optical system. However, in case a radiation having a wavelength shorter than the i-line is used for such optical glasses, the internal transmittance for such a radiation abruptly decreases, and in case of radiations having a wavelength not longer than 250 nm, in particular, such optical glasses can no longer exhibit transmittance for the radiations. Thus, as a material for use in an optical member for use in an optical system of a projection aligner equipped with a light source emitting radiations in the ultraviolet region not longer than 400 nm in wavelength, generally employed is a highly uniform silica glass or a single crystal of calcium fluoride that yield high transmittance for radiations in the ultraviolet region. These two materials are required in case of correcting color aberration in the imaging optics of excimer lasers.
Of the optical materials above, a silica glass not only yields a high optical transmittance, but also has superior properties, for instance, it is characterized in that it has an excellent resistance against excimer lasers; that it yields stability to change in temperature; that it has excellent corrosion resistance and elastic properties; and that it has small linear expansion coefficient (about 5.5xc3x9710xe2x88x927/K) at temperatures in the vicinity or room temperature. Thus, attempts are being made to apply the silica glass as a material constituting the optical members such as reticles, in which some optical properties are required when used in projection aligners, such that it has an excellent UV durability, and that it generates less heat and thereby low thermal expansion occurs.
The application of a single crystal of calcium fluoride to such optical members is being studied, because it has high optical transmittance and an excellent resistance against ultraviolet radiations when used as a material of an optical member, particularly, for radiations having a specified wavelength of 190 nm or shorter.
However, even in an optical member such as a lens, a photomask substrate, etc., made of silica glass, it was found to yield insufficient optical transmittance or resistance against ultraviolet radiations for light having a specified wavelength of 250 nm or shorter. As described above, this is ascribed to the fact that the optical transmittance of the optical system which consists of a plurality of lenses (a group of lenses) as a whole is the accumulation of each of the optical transmittances of the lenses, and that there are problems such as an increase in transmitting loss of the silica glass attributed to the internal absorption and the internal scattering of the light, a generation of color centers due to laser induction, a decrease in optical performance due to heat generation and phosphorescence, a change in density due to compaction, etc. This tendency becomes particularly distinct in case the optical system is used with a light having a wavelength of 190 nm or shorter.
More specifically, when the optical members were used in a projection aligner using a light source such as an ArF excimer laser (having a wavelength of 193 nm), a F2 laser (having a wavelength of 157.6 nm), etc., there was found to induce a problem of line width deviation and the like in the pattern transfer process, thereby making it extremely difficult to achieve a high resolution.
Furthermore, a single crystal of calcium fluoride was disadvantageous in that it suffered breakage during the process of forming the patterns, because it is brittle and easily subject to flaws due to an inferior stability to a change in temperature. Furthermore, it is also a disadvantage for a single crystal of calcium fluoride that it is difficult to form mask patterns at high precision and that it required a strict control of temperature during the exposure treatment, because the linear expansion coefficient thereof is as high as about 40 times as that of a silica glass.
As described above, with decreasing wavelength of the radiation utilized in the projection aligners, still higher optical performance is being required to the optical members such as lens members, photomask members, etc. however, an optical member having the desired optical performance suitable for use in the optics constituting the device utilizing radiations having a wavelength of 250 nm or shorter, and particularly those having a wavelength of 190 nm or shorter, is yet to be developed. Moreover, even in case of using a material having a high transmittance and a high uniformity in refractive index, the optical system constructed by assembling a plurality of such materials sometimes resulted in a failure of obtaining the desired resolution.
Accordingly, an object of the present invention is to provide a silica glass member having high optical transmittance and resistance against ultraviolet radiation, and a method for producing the same, as well as to provide a projection aligner capable of obtaining high resolution.
In the light of such circumstances, the present inventors extensively conducted studies to achieve the object above, and first investigated the factors affecting the birefringence that occurs within the optical member. Conventionally, the birefringence that occurs within the optical member has been believed to be established by the influence of thermal strain that generates inside the optical member during its cooling process after the thermal treatment. However, the present inventors have found that, particularly in case of a silica glass member, the structural distribution (the distribution of SiO2 bonds) and the impurity distribution greatly affect the generation of birefringence.
Hence, the present inventors have studied the impurities inside the silica glass member in detail, and, as a result, it has been found that, among the structural distribution and the impurities incorporated inside the silica glass member, particularly the distribution in the concentration of hydroxyl groups had the greatest influence on the distribution of birefringence generating inside the silica glass member. Furthermore, the present inventors have studied the cooling process (the process of decreasing temperature) after the thermal treatment, and have found that the cooling rate in the temperature range of from 1500 to 1800xc2x0 C. was the determining factor in the degree of birefringence generating inside the silica glass in case it was attempted to obtain silica glass members having low birefringence.
In addition to above, the present inventors have found that the birefringence of the material constituting the optical member greatly affects the image forming performance of the projection optical system and the resolution of the projection aligner. Then, it has been also found that an imaging performance near to the designed performance of the projection optical system and the resolution near to the designed performance for the projection aligner can be obtained if the degree of the birefringence of the material constituting the optical member, more specifically, if the birefringence value (in absolute value) is 2 nm/cm or lower and if the distribution of the birefringence within the optical member is symmetrical with respect to the center. This is disclosed in JP-A-Hei8-107060 (wherein, the term xe2x80x9cJP-A-xe2x80x9d as referred herein signifies xe2x80x9can unexamined published Japanese patent applicationxe2x80x9d).
However, with a further requirement for a higher resolution of a projection aligner, there are cases in which a light having a shorter wavelength is used as the light source or a thicker optical member having a larger aperture diameter is employed. In such cases, even though the design philosophy as above should be employed, it was not always possible to obtain favorable imaging performance of the projection optical system and a fair resolution of the projection aligner.
Accordingly, the present inventors further proceeded with the study, and as a result, the reason why it was not possible to obtain the desired projection optical system or projection aligner having the desired optical performance was obtained even in case an optical member having a favorable transmittance or a favorable uniformity in the distribution of refractive index was used. That is, since the state of distribution of the birefringence within the optical member differs from an optical member to another, the optical system obtained by assembling a plurality of optical members as a whole generates a disturbance in the optical wavefront due to the accumulation of different birefringence, and that this greatly affects the imaging performance of the projection optical system or the resolution of the projection aligner.
In other words, in the conventional evaluation of birefringence of an optical member, only the dimension (in absolute value) of the birefringence was discussed, and furthermore, there was no concept of the distribution in birefringence values within the optical member. For instance, in case of measuring the birefringence value of a silica glass member, it was customary in the art to measure the birefringence value on several points located in the vicinity of a distance corresponding to 95% of the diameter of the member, and to adopt the maximum value as the birefringence value of the member. However, in a detailed measurement of the distribution of the birefringence values on a plurality of measuring points within the silica glass member, the present inventors have found that the birefringence values exhibited a non-uniform distribution.
Accordingly, even in case of a silica glass member having a highly uniform refractive index, the influence of birefringence within the member cannot be sufficiently evaluated by a simple control of the maximum value among the several birefringence values measured on several points, and, in particular, it was found extremely difficult to obtain an optical system having the desired performance in case a plurality of members are assembled. Such a non-uniform distribution in birefringence values within the silica glass member is presumably attributed to the temperature distribution at the synthesis, the non-uniform distribution of impurities, or the non-uniform distribution of the structural defects of SiO2 so that the non-uniform distribution in birefringence values is formed within the silica glass member during the cooling of the member.
As described above, since the overall evaluation of birefringence of the optical system constructed from a plurality of optical members cannot be simply represented by the dimension of birefringence of each optical members, the present inventors performed a detailed study on how the non-uniform distribution in birefringence values within the optical members influence the optical system. As a result, in case of paying particular attention of the non-uniform distribution of the birefringence values in the direction of the fast axis, it has been found for the first time that the birefringence values are accumulated when the optical system is constructed by using the optical members having distribution of the birefringence values having the same fast axis direction, and that this negatively affects the performance of the optical system. It has been also found for the first time that, by assembling optical members having different fast axis directions, the negative influence of the birefringence is cancelled out in the optical system as a whole. Thus, the present inventors accomplished the present invention by also taking into account the knowledge on the influence of the concentration of hydroxyl groups that are present in the silica glass member, which was described hereinbefore on the birefringence generating within the silica glass member.
More specifically, the silica glass member according to the present invention is characterized in that it is a silica glass member for use with a light having a specific wavelength of 250 nm or shorter, in which the difference in the maximum and the minimum values of hydroxyl group concentration as measured in a plurality of points within a plane vertical to an optical axis whose center is the crossing point of its optical axis with the optical axis of the silica glass member is 50 ppm or lower; and in which the plurality of signed birefringence values obtained based on the birefringence values measured on several points within a plane vertical to an optical axis whose center is the crossing point of its optical axis with the optical axis of the silica glass member and the direction of the fast axis fall within a range of from xe2x88x922.0 to +2.0 nm/cm.
A silica glass member having such a distribution in difference between the maximum and the minimum values of hydroxyl group concentration within a plane vertical to an optical axis whose center is the crossing point of its optical axis with the optical axis of the silica glass member of 50 ppm or lower, and in which the plurality of signed birefringence values obtained based on the birefringence values measured on several points within a plane vertical to an optical axis whose center is the crossing point of its optical axis with the optical axis of the silica glass member and the direction of the fast axis fall within a range of from xe2x88x922.0 to +2.0 nm/cm yields a high optical transmittance and a high resistance to ultraviolet radiation. Accordingly, in case a lens or a photomask substrate using the silica glass member according to the present invention is applied to constitute the optical system of a projection aligner, the drop in optical transmittance of the entire optical system can be suppressed; furthermore, in case the silica glass member according to the present invention is applied to a photomask substrate, a projection aligner having a high resolution free from line width deviation in a pattern transfer step can be implemented because not only a high optical transmittance can be obtained, but also the generation of birefringence attributed to a local thermal expansion of the member can be suppressed.
Further, the method for producing a silica glass according to the present invention is characterized in that the method comprises: a synthetic step of silica glass bulk comprising, in a synthetic furnace equipped with a burner having a plurality of tubes, ejecting a raw material and a combustion gas from the plurality of tubes of said burner to hydrolyze said raw material in an oxyhydrogen flame, and thereby synthesizing a silica glass bulk having a difference in the maximum and the minimum values of hydroxyl group concentration as measured in a plurality of points within a predetermined internal plane of the bulk of 50 ppm or lower; a step of cooling silica glass bulk, comprising cooling said silica glass bulk under a pressure range of from 0.01 to 0.15 MPa (abs) while controlling the cooling rate in the temperature region of from 1500 to 1800xc2x0 C. to a range of from 5 to 10xc2x0 C./min; a step of cutting silica glass bulk, comprising cutting out said silica glass bulk to obtain a silica glass member having the desired shape and size; and a step of heat treating silica glass member, comprising applying a heat treatment to said silica glass member to obtain a silica glass member in which a difference in the maximum and the minimum values of hydroxyl group concentration as measured in a plurality of points within a plane vertical to an optical axis whose center is the crossing point of its optical axis with the optical axis of the silica glass member is 50 ppm or lower, and in which the plurality of signed birefringence values obtained based on the birefringence values measured on several points within a plane vertical to an optical axis whose center is the crossing point of its optical axis with the optical axis of the silica glass member and the direction of the fast axis fall within a range of from xe2x88x922.0 to +2.0 nm/cm.
The distribution in the concentration of hydroxyl groups within the silica glass member, which greatly influences the distribution in birefringence within the silica glass member, can be controlled by properly adjusting the type of the gases (raw material, oxygen or hydrogen) ejected from each of the tubes of the multi-tubular burner and the ejecting conditions thereof such as the flow rate, etc., while also properly controlling the specific synthetic treatments applied thereto in accordance with the various synthetic methods for silica glass bulk. For instance, in a direct method, the distribution in the concentration of hydroxyl groups can be minimized by controlling the rocking width of the target. In case of soot method, the distribution in the concentration of hydroxyl groups can be minimized by suppressing the maximum hydroxyl group concentration to 50 ppm or lower.
Furthermore, after controlling the distribution in the concentration of hydroxyl groups within the silica glass member during the synthetic step of silica glass bulk as described above, the cooling rate in the temperature range of from 1500 to 1800xc2x0 C., i.e., in a high temperature region not controlled heretofore, is controlled to a range of from 5 to 10xc2x0 C./min during the cooling step of silica glass bulk, and the conditions of the heat treatment applied to the silica glass member obtained after the step of cutting the silica glass bulk are properly controlled. In this manner, the distribution of the impurities incorporated into the silica glass member and the structural distribution of SiO2 can be homogenized. As a result, there can be realized a silica glass member having a high optical transmittance and a high resistance against ultraviolet radiation, having a plurality of signed birefringence values obtained based on the birefringence values measured on several points within a plane vertical to an optical axis whose center is the crossing point of its optical axis with the optical axis of the silica glass member (and the direction of the fast axis) falling within a range of from xe2x88x922.0 to +2.0 nm/cm.
Conventionally, the rate of decreasing the temperature in the cooling step of cooling the silica glass bulk was commonly set at 15xc2x0 C./min or higher to prevent devitrification from occurring. However, in the method for producing a silica glass member according to the present invention, the cooling rate, particularly in the temperature region of from 1500 to 1800xc2x0 C., was controlled to a range of from 5 to 10xc2x0 C./min to avoid devitrification from occurring, while carrying out the cooling under a pressing condition in a pressure region of from 0.01 to 0.15 MPa to prevent the generation of strain within the silica glass member, thereby realizing uniform distribution in the impurities incorporated into the silica glass member and uniform structural distribution of SiO2.
Further, the method for producing a silica glass according to the present invention is characterized in that the method comprises: a synthetic step of silica glass bulk comprising, in a synthetic furnace equipped with a burner having a plurality of tubes, ejecting a raw material and a combustion gas from the plurality of tubes of said burner to hydrolyze said raw material in an oxyhydrogen flame, and thereby synthesizing a silica glass bulk having a difference in the maximum and the minimum values of hydroxyl group concentration as measured in a plurality of points within a predetermined internal plane of the bulk of 50 ppm or lower; a step of cooling silica glass bulk, comprising, while holding said silica glass bulk inside said synthetic furnace, cooling said silica glass bulk based on the temperature difference between the silica glass bulk and the temperature of the external environment; a step of cutting silica glass bulk, comprising cutting out said silica glass bulk to obtain a silica glass member having the desired shape and size; a first heat treatment step of silica glass member, comprising elevating the temperature of said silica glass member to a predetermined temperature in the temperature range of from 1600 to 2300xc2x0 C. in an inert gas atmosphere in a pressure range of from 0.01 to 0.15 MPa (abs); and a second heat treatment step of silica glass member, comprising, in an inert gas atmosphere maintained in a pressure range of from 0.01 to 0.15 MPa (abs), cooling said silica glass member while controlling the cooling rate in the temperature region of from 1500 to 1800xc2x0 C. to a range of from 5 to 10xc2x0 C./min to obtain a silica glass member in which a difference in the maximum and the minimum values of hydroxyl group concentration as measured in a plurality of points within a plane vertical to an optical axis whose center is the crossing point of its optical axis with the optical axis of the silica glass member is 50 ppm or lower, and in which the plurality of signed birefringence values obtained based on the birefringence values measured on several points within a plane vertical to an optical axis whose center is the crossing point of its optical axis with the optical axis of the silica glass member and the direction of the fast axis fall within a range of from xe2x88x922.0 to +2.0 nm/cm.
The method for producing a silica glass member above comprises producing a silica glass bulk to which the control of distribution in the concentration of internal hydroxyl groups alone is applied, then, applying a heat treatment to a prototype of a member obtained by cutting out the silica glass bulk. Also in this case, the distribution of the impurities incorporated into the silica glass member and the structural distribution of SiO2 can be homogenized by elevating the temperature of the prototype of the member under the conditions above, and then controlling the cooling rate in the temperature region of from 1500 to 1800xc2x0 C. in a range of from 5 to 10xc2x0 C./min. As a result, there can be realized a silica glass member having a high optical transmittance and a high resistance against ultraviolet radiation, having a plurality of signed birefringence values obtained (based on the birefringence values measured on several points) within a plane vertical to an optical axis whose center is the crossing point of its optical axis with the optical axis of the silica glass member (and the direction of the fast axis) falling within a range of from xe2x88x922.0 to +2.0 nm/cm.
In addition, the projection aligner according to the present invention is characterized in that it comprises an exposure light source emitting a light having a wavelength of 250 nm or shorter as the exposure light, a reticle having formed thereon an original pattern image, an irradiation optical system which irradiates a radiation output from said exposure light source to said reticle, a projection optical system which projects a pattern image output from said reticle onto a photosensitive substrate, and an alignment system which aligns said reticle and said photosensitive substrate, in which at least a part of the optical member constituting said irradiation optical system, the optical member constituting said projection optical system, and said reticle, is made of a silica glass member according to the present invention.
The projection aligner according to the present invention realizes an excellent resolution by implementing it with an optical system constructed from a silica glass member according to the present invention.
The concept of xe2x80x9csigned birefringence valuexe2x80x9d as referred in the present invention is explained below.
A signed birefringence value is, a birefringence value having a sign attached thereto by taking into consideration the direction of the fast axis defined in the index ellipsoid used in case of obtaining the birefringence value of an optical member.
More specifically, an effective cross section is taken as the region receiving the circular irradiation of a radiation flux within a plane vertical to an optical axis whose center is the crossing point of its optical axis with the optical axis of the optical member and the direction of the fast axis; the sign is plus (or minus) if the direction of the fast axis in a minute region of the point of measuring the birefringence on the effective cross section is parallel with the direction of radiation from the center corresponding to the crossing point with the optical axis of the optical member, and is minus (or plus) if the direction above is vertical with said direction of radiation.
The above concept of attaching a sign to the birefringence value can be applied to a case a plurality of radiation fluxes are irradiated to a plane vertical to the optical axis having its center at the crossing point with the optical axis of the optical member. In this case again, a plus (or minus) sign is attached to the birefringence value measured if the direction of radiation from the center corresponding to the crossing point with the optical axis of the optical member is parallel with the fast axis direction within a minute region of the point of measuring the birefringence taken on each of the effective cross sections on which a plurality of radiation fluxes are irradiated, or a minus (or plus) sign is attached if the direction above is vertical.
Furthermore, the above concept of attaching a sign to the birefringence value can be applied to a case a radiation flux having a cross section other than a circular shape is irradiated on the plane vertical to the optical axis having its center at the crossing point with the optical axis of the optical member. For instance, it is applicable to a case a radiation flux having a ring cross section or a ellipsoidal cross section is irradiated. Similarly in this case, a plus (or minus) sign is attached to the birefringence value measured if the direction of radiation from the center corresponding to the crossing point with the optical axis of the optical member is parallel with the fast axis direction within a minute region of the point of measuring the birefringence taken on each of the effective cross sections on which a plurality of radiation fluxes are irradiated, or a minus (or plus) sign is attached if the direction above is vertical.
In the explanation below, a plus sign is attached to the measured birefringence value if the direction of radiation from the center corresponding to the crossing point with the optical axis of the optical member is parallel with the fast axis direction within a minute region of the point of measuring the birefringence taken on the effective cross sections on which radiation fluxes are irradiated, and a minus sign is attached if the above directions are vertical to each other.
The signed birefringence value is described more specifically below by making reference to FIG. 1A, FIG. 1B, FIG. 2A, FIG. 2B, FIG. 3A, and FIG. 3B.
FIG. 1A is a schematic diagram showing the direction of the fast axis at the points for measuring birefringence P11, P12, P13, and P14, each located at a distance of r1, r2, r3, and r4 from the center O of the effective cross section taken on an optical member L1. In the figure, for the convenience of explanation, the points for measuring birefringence P11 to P14 are set on a straight line Q1 passing through the center O1 and being extended along the direction of the radius. In the figure, the size of the minute region expressed by the circles at each of the measuring points corresponds to the optical path difference at each of the measuring points. Furthermore, the direction of the linear segments within the minute regions W11, W12, W13, and W14 corresponds to the direction of the fast axis. Since the direction of the fast axis at the measuring points P11 to P14 is in parallel with the direction of the straight line Q1, i.e., the direction of the radius, the birefringence values for the measuring points P11 to P14 are all expressed with a plus sign attached thereto. On drawing the distribution of the signed birefringence values A11, A12, A13, and A14 along the direction of the radius for the measuring points P11 to P14 obtained in FIG. 1A, for instance, a profile as shown in FIG. 1B can be obtained.
Similar to FIG. 1A, FIG. 2A is a schematic diagram showing the direction of the fast axis at the points for measuring birefringence P21, P22, P23, and P24, each located at a distance of r1, r2, r3, and r4 from the center O2 of the effective cross section taken on an optical member L2. Since the direction of the fast axes W21, W22, W23, and W24 at the measuring points P21 to P24 is vertical to the direction of the straight line Q2, i.e., the direction of the radius, the signed birefringence values A21, A22, A23, and A24 for the measuring points P11 to P14 are all expressed with a minus sign attached thereto. On drawing the distribution of the signed birefringence values A21 to A24 along the direction of the radius for the measuring points P21 to P24 obtained in FIG. 2A, for instance, a profile as shown in FIG. 2B can be obtained.
Similarly as in FIG. 1A, FIG. 3A is a schematic diagram showing the direction of the fast axis at the points for measuring birefringence P31, P32, P33, and P34, each located at a distance of r1, r2, r3, r4 and r5 from the center O of the effective cross section taken on the optical member L2. In the measuring points P11, to P14, the direction of the fast axes W31, W32, W33, W34 and W35 at the measuring points P31 to P33 is in parallel with the direction of the straight line Q3, i.e., the direction of the radius, whereas the direction at points P34 and P35 is vertical to the direction of the radius. Thus, on drawing the distribution of the signed birefringence values A31 to A35 along the direction of the radius for the measuring points P31 to P35 obtained in FIG. 3A, a profile as shown in FIG. 3B can be obtained.
FIG. 4A shows a schematically drawn side view of the m silica glass members constituting the optical system, being arranged sequentially from the light source. FIG. 4B is a schematically drawn cross section view showing the effective cross section vertical to the optical axis of one of the m silica glass members shown in FIG. 4A, i.e., the silica glass member Li arranged at the ith position from the light source.
In the present invention, let the distribution of the birefringence values within the silica glass member be uniform within the direction of thickness of a member in parallel with the direction of optical axis, and let the distribution of the birefringence values in the direction of the radius in the effective cross section perpendicular to the optical axis be non-uniform. The term xe2x80x9ceffective cross sectionxe2x80x9d as referred herein shows the region receiving the irradiation of light flux within the plane perpendicular to the optical axis of the silica glass member. Thus, the crossing point with the optical axis is taken as the center of the effective cross section, and the radius corresponds to the effective radius of the effective cross section of the silica glass member. Furthermore, since the size of the effective cross section differs depending on the silica glass members in case of measuring the signed birefringence values of the entire optical system, the size of the effective cross section of all of the silica glass members is normalized as such that that the maximum effective radius rn of the silica glass member should become 1 as shown in FIG. 4A.
Further, in case a plurality of light fluxes are irradiated to the plane vertical to the optical axis having a center corresponding to the crossing point of the optical axis of the silica glass member, the size of the effective cross section of all of the silica glass members is normalized as such that the maximum effective radius rn corresponding to each of the light fluxes should become 1.
Furthermore, in case a light flux yields a cross section with a shape other than a circular one on the plane vertical to the optical axis having a center corresponding to the crossing point of the optical axis of the silica glass member, for instance, a light flux having a ring cross section or an ellipsoidal cross section is irradiated, the size of the effective cross section of all of the silica glass members is normalized in beforehand as such that each of the silica glass members yield a maximum effective radius rn corresponding to each of the light fluxes should become 1.
For instance, in case a light flux having a ring cross section is irradiated, the size of the effective cross section of the entire silica glass member is normalized previously in such a manner that the maximum outer diameter of the ring should become 1, and the measurement of the signed birefringence value can be performed in accordance with the method described below in the same manner as the measurement for a light flux having a circular cross section. In case of a light flux having an ellipsoidal cross section is irradiated, the size of the effective cross section of the entire silica glass member is normalized previously in such a manner that the maximum outer diameter for the major axis of the ellipsoid should become 1, and the measurement of the signed birefringence value can be performed in accordance with the method described below in the same manner as the measurement for a light flux having a circular cross section.
To measure the signed characteristic birefringence value of the entire projection system, firstly, with reference to FIG. 4B, a model is set as such that it consists of a plurality of concentrical circles Cij having a center Oi on the effective cross section of one of the optical members, Li, and that said concentrical circles each differing in radius from the center. Then, the birefringence value at a kth measuring point Pijk located on the jth concentrical circle Cij having a radius of rj from the center Oi is measured. Subsequently, a sign is attached by determining the sign from the relation between the direction of the fast axis at the measuring point Pijk and the direction of the radius to obtain a signed birefringence value Aijk.
In this case, i represents the number of the optical member L above constituting the projection optical system (i=1, 2, . . . m; 2xe2x89xa6m). Similarly, j represents the number of concentrical virtual circles C present on an effective cross section vertical to the optical axis of said an optical member L, each having a center on said optical axis and different from each other in radius from said optical axis (j=1, 2, . . . n; 1xe2x89xa6n). Furthermore, k represents the number of measuring points located on the circumference of said concentrical circles C (k=1, 2, . . . h; 1xe2x89xa6h). In this manner, the signed birefringence values Aijl to Aijh are measured for the predetermined measuring points Pijl to Pijh located on the same concentrical circle Cij.
Then, the mean signed birefringence value Bij, which is the arithmetic mean of the signed birefringence values of the measuring points located on the circumference of the concentrical circle Cij of an optical member Li, is calculated in accordance with the following equation:                               B          ij                =                                            ∑                              k                =                1                            b                        ⁢                          xe2x80x83                        ⁢                          A              ijk                                h                                    (2)            
Thus, in case a silica glass member yields a mean signed birefringence value Bij satisfying the relation below, the silica glass member exhibits high optical transmittance and resistance against ultraviolet radiations; furthermore, an optical system utilizing such silica glass members as a whole yields an excellent imaging performance, and the projection aligner equipped with such a projection system exhibits excellent resolution:
xe2x88x922.0xe2x89xa6Bijxe2x89xa62.0 nm/cmxe2x80x83xe2x80x83(1).
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.